Magnetic resonance imaging (MRI) is a noninvasive imaging technique that provides clinicians and diagnosticians with information about the anatomical structure and condition of a region of interest within a subject. See, for example, U.S. Pat. No. 5,671,741 to Lang et al. issued Sep. 30, 1997 for “Magnetic Resonance Imaging Technique for Tissue Characterization;” U.S. Pat. No. 6,219,571 B1 to Hargreaves et al. issued Apr. 17, 2002, for “Magnetic Resonance Imaging Using Driven Equilibrium Fourier Transform;” U.S. Pat. No. 6,479,996 to Hoogeveen et al. issued Nov. 12, 2002 for “Magnetic Resonance Imaging of Several Volumes;” U.S. patent application Ser. No. 2002/0,087,274 A1 to Alexander et al. published Jul. 4, 2002 for “Assessing the Condition of a Joint and Preventing Damage.” Commonly, in MRI, a substantially uniform temporally constant main magnetic field (B.sub.0) is set up in an examination region in which a subject being imaged or examined is placed. Via radio frequency (RF) magnetic field (B.sub.1) excitation and manipulations, selected magnetic dipoles in the subject that are otherwise aligned with the main magnetic field are tipped to excite magnetic resonance. The resonance is typically manipulated to induce detectable magnetic resonance echoes from a selected region of the subject. In imaging, the echoes are spatially encoded via magnetic gradients set up in the main magnetic field. The raw data from the MRI scanner is collected into a matrix, commonly known as k-space. By employing inverse Fourier, two-dimensional Fourier, three-dimensional Fourier, or other known transformations, an image representation of the subject is reconstructed from the k-space data.
Conventional MRI scans produce a data volume, wherein the data volume is comprised of voxels having three-dimensional characteristics. The voxel dimensions are determined by the physical characteristics of the MRI machine as well as user settings. Thus, the image resolution of each voxel will be limited in at least one dimension, wherein the loss of resolution in at least one dimension may lead to three-dimensional imaging problems.
There are many applications in which depth or three-dimensional (“3D”) information is useful for diagnosis and formulation of treatment strategies. For example, in imaging blood vessels, cross-sections merely show slices through vessels, making it difficult to diagnose stenosis or other abnormalities. Likewise, interventional imaging, such as needle tracking, catheter tracking, and the like, requires 3D information. Also, depth information is useful in the so-called interactive imaging techniques in which images are displayed in real or near-real time and in response to which the operator can adjust scanning parameters, such as view angle, contrast parameters, field of view, position, flip angle, repetition time, and resolution.
Three-dimensional imaging generally involves either acquiring multiple two-dimensional or slice images that are combined to produce a volumetric image or, alternately, the use of three-dimensional imaging techniques. Much effort at improving the efficiency of volume imaging has been focused on speeding up the acquisition. For example, many two-dimensional fast scan procedures have been adapted to three-dimensional imaging. Likewise, efforts have been made to improve reconstruction speed and efficiency, for example, through the use of improved reconstruction algorithms. Nevertheless, three-dimensional imaging remains relatively slow.
However, current MRI acquisition techniques do not provide high resolution in all planes and quantitative image analysis using isotropic or near-isotropic imaging. Accordingly, the present invention contemplates new and improved magnetic resonance imaging techniques.
An additional problem not addressed by current 3D MRI scanning methods is the reduction of partial volume effects. Partial volume effects are caused when a voxel falls within the boundary between two scanned objects. For example, if a patient's knee is being sagittally scanned, a voxel may be orientated such that part of the voxel falls within the femur and part falls within a space outside of the femur. MR imaging will average the overall gray value over the entire voxel. The lower the scanning resolution the greater the partial volume effects. In a 3D scan, where there is low resolution in at least one plane of the scan impact of the partial volume effects is greatly increased. Thus, there is a need for methods of forming 3D MRI scans with reduced impact of partial volume effects.
Still further, an additional shortcoming of conventional 3D MRI scanning procedures is that boundaries of scanned objects may be missed due to scanning resolution and scan orientation. This may occur when a boundary of an object being scanned lies between the slice thickness of the scan or the boundary of an object is parallel to the imaging plane. Therefore there is a need for improved methods for reducing the likelihood of missed boundaries.